JRA (also known as JIA) includes the commonest chronic autoimmune arthropathies of childhood. The MHC is involved with respect to risk, either susceptibility or protection in a subtype specific manner with strong gender bias'and differences between ethnicities. Multiple MHC effects have been shown, especially in the commonest subtype, so called early onset pauciarticular JRA (Persistent Oligo in the JIA terminology) with three or more MHC regions believed to interact in generating susceptibility. An additional feature of the disease, unlike some other forms of autoimmunity, is the relative absence of common extended or ancestral haplotypes, especially those carrying HLA-DR4 and HLA-DR7 both of which are protective. The three regions include a class I region, or an area telomeric to it, and two class II regions those around HLA DR/DQ and HLA-DP. None of the regions involved are well defined nor were the specific genes involved identified. The alleles marking these regions (HLA-DR8, 11 and HLA-DPB1*0201) are atypical for autoimmunity. This is therefore an unusual MHC contribution to autoimmunity, the elucidation of which lends itself to high throughput technologies. The genetic features, although involving arthritis, are quite distinct from adult rheumatoid arthritis except for about 5% of older children. It is proposed to construct high throughput SNP maps in a family based study. Subtypes have different MHC profiles and in the rarest and most severe form of disease, systemic onset JRA, the MHC effect is rather minimal. In this form, preliminary data involving KIR gene haplotypes is available. Pursuing these KIR gene observations is proposed. The ability to leverage ongoing pheontyping and family based sample collection ensures a large and continuously growing pool of available DMAs for this project. Some of the patients will also have extensive gene expression studies allowing a comprehensive approach to the MHC and KIR genes in JRA and its subtypes.